1. Field of the Invention
Liquid development of latent electrostatic images.
2. Description of the Prior Art
In the art of development of latent electrostatic images to which the present invention pertains, the first step is to form on a carrier a latent electrostatic image which consists of many electrostatic segments in various configurations such, for instance, as lines forming characters, letters, symbols, etc., or an object or a picture, the electrostatic segments being juxtaposed to uncharged segments of the carrier. The charged segments can be created in various fashions such, for example, as providing a carrier with a photoconductor insulating layer, imposing an electrostatic charge on the layer, and subsequently discharging the electrostatic charge on segments of the layer by exposure of the layer to a modulated beam of radiant energy, leaving an electrostatically charged latent image. Various other methods can be employed to form a latent electrostatic image such, for example, as providing a carrier which is a dielectric sheet and transferring a preformed electrostatic charge to this sheet.
The current most widely used system for developing a latent electrostatic image, particularly for so-called "office copier" machines, is that in which a developer is cascaded across the latent image. The developer is a dry developer and constitutes a powder which coats carrier particles and is adhered thereto by electrostatic forces that usually are generated triboelectrically. The powder is preferentially attracted from the carrier particles to the latent image, thereby leaving powder on the latent image but not on the uncharged segments. The powder includes a thermoplastic material in the most widely used office copiers. The latent image is carried by a photoconductor surface other than the ultimate copy sheet such, for instance, as a selenium drum, and the powder, in the configuration of the ultimate image, is transferred from the drum to the copy sheet to which it is made to adhere by the application of heat sufficient to fuse the powder to the sheet but insufficient to discolor the sheet.
It also has been proposed to use a flexible copy sheet coated with a photoconductor insulating layer as the carrier for the electrostatic image and to fuse the powder directly to such a sheet.
It further has been proposed to develop a latent electrostatic image by the use of a powder cloud, the developer powder being suspended in a gas which is blown against the latent image and selectively will deposit on the image largely to the exclusion of deposit on the uncharged segments.
All the foregoing techniques are subject to various mechanical drawbacks such as intricate cascading equipment, intricate recirculating equipment for the toner to capture the toner which cascades off the photoconductor insulating layer and bring it back to a zone at which the cascading takes place, and intricate fusing equipment. The powder cloud developing method never has reached the point of widespread commercial acceptance. A further problem with the dry developers was that the powder coating on the carrier particles frequently became dislodged during the cascading across the latent electrostatic image and adhered, for various reasons, to uncharged portions of the carrier, thus forming distracting deposits in the background where no such deposits should be present.
An alternate type of development which basically distinguishes from the dry development above described also has come into vogue, although not to the extent of dry development. This is the so-called "liquid" developing machine. Liquid developing machines have been of various types. That most frequently used is one in which the electrostatic image is formed on a flexible carrier sheet having a photoconductor insulating coating such, for example, as zinc oxide in a carrier such as polyvinyl acetate. The sheet with the latent image thereon then has applied to the image-carrying surface thereof a liquid developer which has most commonly constituted an organic liquid of a high resistivity, as well as dispersed coloring developer particles. Where the liquid is in contact with the image, it conventionally has been in contact with all or substantial portions of the image. The colored developer particles dispersed in the liquid are electrostatically attracted to the sundry image segments of the latent image on the carrier traveling through the liquid and mass at the charged image segments where they remain after the carrier leaves the body of the liquid. The particles may be thermoplastic and fixed to the carrier by the application of sufficient heat to fuse the particles without damaging the carrier. Surface adsorption and/or penetration may be factors in fixing the particles to the carrier in image form. The organic liquid evaporates to leave a dry developed image.
The equipment for a liquid developer machine is considerably simpler than that for a dry developer machine, but the liquid developer machine has many drawbacks such as the loss of the organic liquid, the necessity for ventilating the room in which the equipment is located in order to prevent an objectionable concentration of the vapor of the organic liquid, and the tendency of the uncharged segments of the image to attract some of the developer particles which results in a discoloration of the background and a loss of contrast as well as a loss of edge definition of the developed image.
Other problems with such liquid developing equipment are the creation, on occasion, of a trailing deposit of the particles beyond the edges of the developed image segments in the direction of travel of the carrier through the equipment, an inability to deposit enough particles from the liquid where a charged segment is of substantial dimension, for example, exceeding 1/4" in any direction, and the susceptibility of the image to smudging due, in part, to incomplete setting of the image because of residual retention of the organic liquid.
Another drawback of the liquid developing equipment was economic, and constituted the rather high cost of preparing a liquid developer. The particle size had to be extremely small, the dispersion of the particles in the organic liquid had to be substantially uniform, the particles had to be prevented from settling either upon standing in the machine between uses and over weekends or on the shelf in a storehouse or at a distributor's, or even on the consumer's premises. The drawback of settling of the particles has been overcome by some manufacturers by the inclusion of dispersion agents, and by other manufacturers by special formulation of the solid contents of the liquid developer which made the coloring agents part of an amphipathic molecule of which another part was soluble in the organic liquid. None of these improvements substantially reduced the cost of liquid developers.
It has been proposed to reduce the expense of providing a liquid developer by using water as a carrier in place of the organic liquid. This, also, of course, reduces the problem of accumulation of vapors of the organic liquid in the vicinity of the office copier. It also reduces the cost of the liquid developer. The liquids proposed, generally, have been water containing a water-soluble dye. Due to the employment of the dye as the coloring agent, the problem of settling and dispersion was eliminated. However, to date, there have not been proposed any liquid developing systems employing an aqueous carrier which systems have been commercially accepted. The principal problem with these systems has been that of deposition of some of the liquid developer on the uncharged portion of the photoconductor, with consequent loss of a clean background.
Many such systems have been suggested. One was that disclosed in U.S.L.P. 3,084,043 in which a liquid developer was provided in the valleys of a surface, e.g. a roller surface including lands. The lands contacted or sometimes were very slightly spaced from the surface of the photoconductor and, due to electrostatic force created by the segments of the electrostatic latent image, crept up the sides of the valleys to contact such segments and, theoretically, did not creep up the sides of the valleys in segments where no charge existed. This system was proposed for both polar and non-polar developer liquids.
In another system using a liquid developer, it was proposed to have a developer roller turn in a tank of developing liquid and to provide an electrostatic field in a gap between the developer roller and the latent-image-bearing photoconductor surface. This force raised a swell, something like a standing wave, of developer, to fill the space between the roller and the photoconductor surface, the image being developed essentially by the same principal as that employed to develop a latent image using a liquid developer in which all or successive portions of the latent image were treated with the developer, i.e. both charged and uncharged segments.
Another method proposed was that disclosed in patents such as U.S. Pat. No. 3,383,209 wherein valleys of a developer roller were partly filled with a developing liquid while the lands of the roller engaged the photoconductor surface bearing the latent image. The theory of this system was that the developing liquid had a surface tension such as to render the liquid normally lyophobic with respect to the latent-image-carrying surface of the photoconductor, with the strength of the electrostatic field created by the segments of the latent image being sufficient to result in wetting with the developer of the portions of the surface having latent image segments thereon.
Still another liquid developing system was proposed in U.S. Pat. No. 3,560,204 in which a developer roller turned in a tray containing a liquid developer, the surface of the roller passing beneath a photoconductor web having a latent electrostatic image thereon. The latent image consisted of charged segments juxtaposed to uncharged segments. The spacing, between the film of developing liquid pulled out of the tray by the roller as it turned and the uncharged segments of the image-bearing photoconductor surface, was quite small, not exceeding 3.2 mils. A bias charge was applied to the developer roller that sufficed to draw up unsupported columns of liquid developer purportedly in the shape of the uncharged segments to form ink images on the photoconductor surface. These columns were said to rupture as the photoconductor surface and the film of liquid developer diverged when the roller turned away from its zone of closest proximity to the photoconductor surface.
The processes of all the prior liquid developer systems have been found to leave marks on the background areas where no coloration, theoretically, should exist, and apparently it is for that reason that these processes have not found wide commercial favor despite the many sophisticated modifications which have been proposed.
It will be appreciated that, in general, the common drawback of the various approaches to developing a latent electrostatic image with a liquid developer is the inability to maintain a virgin and, therefore, unmarked background segment where no color is supposed to exist.
Another liquid developer of the type in which the photoconductor surface bearing a latent image is exposed in its entirety or in part to a liquid is illustrated in U.S. Pat. No. 3.068,115 wherein droplets of a developing liquid are formed and drawn through a dielectric liquid in which the developer droplet is immiscible to said photoconductor which is submerged in the dielectric liquid, the droplets being attracted to the charged segments of the photoconductor. This arrangement, obviously, is unsuitable for commercial purposes since it is bulky, unwieldy, slow, requires different liquids in the same piece of equipment, and requires subsequent drying of the photoconductor.
The foregoing summarizes the main systems known to the inventors, it being appreciated that there are a large number of variants of these systems as well as other systems which are in no way relevant to the present invention.